Cubic boron nitride cutting tool

ABSTRACT

An object is to prolong the life of a cubic boron nitride cutting tool used for cutting a heat-resistant alloy. A cubic boron nitride cutting tool includes an edge tip made of a sintered cubic boron nitride compact including cubic boron nitride particles; and a base metal that holds the edge tip at a corner portion of the base metal, wherein a cutting edge formed on the edge tip of the tool has a positive rake angle.

This application is a continuation application of U.S. patentapplication Ser. No. 14/432,641 filed Mar. 31, 2015, which is in turn aU.S. National Stage of International Application No. PCT/JP2014/058263,filed Mar. 25, 2014, which claims the benefit of Japanese PatentApplication No. 2013-100055 filed May 10, 2013. The disclosure of theprior applications is hereby incorporated by reference herein in theirentirety.

TECHNICAL FIELD

The present invention relates to a cubic boron nitride cutting toolincluding a cutting edge made of a sintered cubic boron nitride (cBN)compact, more specifically, to a life-prolonged cubic boron nitridecutting tool used for high-speed cutting of a heat-resistant alloy, suchas a Ni-base heat-resistant alloy.

BACKGROUND ART

One example of materials hard to cut in the cutting process is a Ni-baseheat-resistant alloy. When a cemented carbide tool that has beenfrequently used in the cutting process of a Ni-base heat-resistantalloy, serving as a workpiece, is used, the cutting speed is restrictedto 80 m/min or lower at the fastest in consideration of the strength ofthe tool, whereby enhancement of the work efficiency is prevented.

Thus, it has been studied to perform cutting at a high speed of 200m/min or higher using a cubic boron nitride cutting tool including acutting edge made of a sintered cubic boron nitride compact that has ahigh hot hardness.

While cubic boron nitride has a higher hardness than cemented carbide,cubic boron nitride has a low toughness. Thus, a lateral cutting edgeportion is chipped when the above-described cubic boron nitride cuttingtool is used to cut a heat-resistant alloy and the cubic boron nitridecutting tool is insufficient for reliable life sustainability. Althoughcubic boron nitride has a higher toughness than ceramics, the lack oftoughness for cutting a heat-resistant alloy is undeniable.

As illustrated in PTL 1 below, a trial conducted as a measure to addressthe above problem is to cut a workpiece while the workpiece is softenedwith the heat (cutting heat) occurring at the edge formed of thesintered cubic boron nitride compact as a result of reduction of thethermal conductivity of the sintered cubic boron nitride compact.

Another measure to toughen the cutting edge is to make the rake angle atthe edge negative by forming a negative rake face or by other means.This measure is effective in preventing chipping of a cutting edge madeof a brittle material.

CITATION LIST Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication No.    2011-189421

SUMMARY OF INVENTION Technical Problem

Cutting edges made of a brittle material having a low toughness areusually subjected to an edge toughening process. The cutting edgessubjected to the edge toughening process (such as forming a negativerake face) reduce their sharpness due to the edges being blunted by thetoughening process, whereby the cutting edges produce a larger amount ofcutting heat.

Thus, blunting the edge has been thought to be effective in prolongingthe life of a tool in the case of cutting a workpiece using a cuttingedge made of a sintered cubic boron nitride compact while the workpieceis softened by the cutting heat.

This measure, however, did not sufficiently prolong the lives of cubicboron nitride cutting tools.

In view of the circumstances, the present invention aims to prolong thelife of a cubic boron nitride cutting tool used for cutting aheat-resistant alloy using a method that has been avoided.

Solution to Problem

To solve the above-described problem, the present invention provides acubic boron nitride cutting tool that includes an edge tip made of asintered cubic boron nitride compact having a thermal conductivitywithin the range of 20 to 70 W/m·K and including cubic boron nitrideparticles having an average particle diameter within the range of 0.5 μmto 2 μm; and a base metal that holds the edge tip at a corner portion ofthe base metal, wherein a cutting edge formed on the edge tip of thetool has a positive rake angle.

Advantageous Effects of Invention

In the cubic boron nitride cutting tool according to the presentinvention, an edge tip is made of a sintered cubic boron nitride compacthaving a low thermal conductivity and a cutting edge formed on the edgetip has a positive rake angle.

Results of experiment have revealed that the cubic boron nitride cuttingtool in which the cutting edge has a positive rake angle has lowerlateral-cutting-edge-portion boundary wear (written as VN, below) orflank face wear (or flank wear, written as VB, below) than existingcubic boron nitride cutting tools used for cutting a heat-resistantalloy that have been subjected to an edge toughening process, wherebythe life of the cubic boron nitride cutting tool has prolonged comparedto the lives of existing tools.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of an example of a cubic boron nitridecutting tool according to the present invention.

FIG. 2 is a perspective view of another example of a cubic boron nitridecutting tool according to the present invention.

FIG. 3 is an enlarged plan view of a corner portion of the cubic boronnitride cutting tool illustrated in FIG. 1 at which the edge tip isdisposed.

FIG. 4 is a cross-sectional view of the corner portion taken along thebisector CL of the corner angle illustrated in FIG. 3.

FIG. 5 is a perspective view of an enlarged edge portion.

FIG. 6 is a plan view of a main portion of an example of a grindingmachine used for manufacturing the tool according to the presentinvention.

FIG. 7 is a front view illustrating the movement of a chuck of thegrinding machine.

FIG. 8 is a plan view illustrating the movement of the chuck of thegrinding machine.

FIG. 9 illustrates the state of grinding the rake face.

DESCRIPTION OF EMBODIMENTS

Referring now to the attached drawings from FIG. 1 to FIG. 9, cubicboron nitride cutting tools according to embodiments of the presentinvention are described below. The present invention is not limited tothese examples and is described by the scope of claim. The presentinvention is intended to include equivalents of the scope of claim andall the modifications within the scope of claim.

FIG. 1 illustrates an example in which the present invention is appliedto a triangular cubic boron nitride cutting insert and FIG. 2illustrates an example in which the present invention is applied to asquare cubic boron nitride cutting insert.

These cubic boron nitride cutting tools (cubic boron nitride cuttinginserts) 1 each include an edge tip 3, made of a piece of a sinteredcubic boron nitride compact, joined at a corner portion of a base metal(shaped in a polygon including a rhombus) 2 made of any of materialsincluding cemented carbide, ceramics, cermet, or sintered alloys. Thesintered cubic boron nitride compact forms a cutting edge 4 of eachcubic boron nitride cutting tool 1. The reference symbol 5 denotes arake face formed in the edge tip 3 and the reference symbol 6 denotes aflank face.

The edge tip 3 is made of a sintered cubic boron nitride compactincluding cubic boron nitride particles having an average particlediameter within the range of 0.5 μm to 2 μm and having a thermalconductivity within the range of 20 to 70 W/m·K determined by adjustingthe content of the cubic boron nitride particles.

Determining the average particle diameter of the cubic boron nitridepowder, serving as a material of the sintered cubic boron nitridecompact, within the range of 0.5 μm to 2 μm and determining the thermalconductivity of the sintered cubic boron nitride compact containing thecubic boron nitride powder within the range of 20 to 70 W/m·K enableboth the formation of a positive rake angle, described below, andprevention of chipping and wear of a lateral cutting edge portion.

The tool illustrated in FIG. 2 includes such edge tips 3 at all thecorner portions. Such edge tips 3 do not necessarily have to be providedat all the corner portions. An edge tip may be provided at one cornerportion, as in the case of the tool illustrated in FIG. 1, or edge tipsmay be provided at two acute diagonal corner portions in the case of arhombus cutting insert.

The edge tip of a sintered cubic boron nitride compact is manufacturedin the method described below. The method is described, for example, inPTL 1 cited above.

The edge tip of a sintered cubic boron nitride compact is manufacturedin the following steps. Firstly, WC powder, Co powder, and Al powder,which have fine particle diameters, are mixed together at a mass ratioof WC:Co:Al=25:68:7. The mixture is thermally treated at 1000° C. for 30minutes under vacuum to form a compound and the compound is ground toobtain material powder constituting a binder phase.

Subsequently, a mixture of Al powder and Zr powder, which also have fineparticle diameters, are thermally treated at 1000° C. for 30 minutes inan atmosphere of nitrogen to obtain a first compound. The first compoundis then roughly ground. Thereafter, using a zirconia medium having adiameter smaller than 1 mm, the roughly ground first compound and themedium are finely ground in an ethanol solvent and then the medium isremoved from the resultant to obtain a material powder constituting aheat-insulating phase.

Subsequently, the obtained material powder constituting the binderphase, the obtained material powder constituting the heat-insulatingphase, and cubic boron nitride powder having an average particlediameter within the range of 0.5 μm to 2 μm are combined and mixed sothat the cubic boron nitride content of the mixture has a desired volume% after sintering and the mixture is dried. The mixed powder is thenstacked on a metal support plate, filled in a Mo capsule, and thensintered by an ultra-high pressure generator under the conditions, forexample, at 7 GPa at the temperature of 1750° C. for 30 minutes.

Thus, a sintered cubic boron nitride compact is obtained. The sinteredcubic boron nitride compact is cut into a predetermined shape and joinedto a base metal to obtain a blank for a tool. Thereafter, some portionsof the resultant are ground as necessary to form a tool having apredetermined shape.

Grinding is performed on at least a rake face 5 and a flank face 6formed on the edge tip 3. When the rake face 5 is ground, the cuttingedge 4 is provided with a positive rake angle. The rake angle θillustrated in FIG. 4 is formed so as to have a positive degree withinthe range of 2° to 20°, more preferably, within the range of 5° to 15°.

The rake angle θ is an angle measured at the foremost end (edge) of thecutting edge. FIG. 4 is a cross section of the corner portion takenalong the bisector CL of a corner angle in the plan view of FIG. 3. InFIG. 3, the position at which the bisector CL of the corner anglecrosses the cutting edge 4 is the foremost end of the cutting edge.

The reason why the rake angle θ is restricted to the above-describedrange is because the results of an evaluation test have revealed thatthe rake angle θ within the range of 5° to 15° has exerted aparticularly high wear resistant effect although the rake angle θ withinthe range of 2° to 5° or 15° to 20° has exerted a certain wear resistanteffect.

Desired portions of a tool are ground using a NC grinding machine formanufacturing the tool. A grinding machine used for manufacturing thetool includes a chuck 11, which is illustrated in FIG. 6 and whoseposition and orientation is numerically controlled, and a grinding wheel(a cup grinding wheel is illustrated) 12 that rotates at a fixedposition.

A workpiece (cubic boron nitride cutting tool) is carried to or from thegrinding machine and handed to or from the chuck 11 by a robot hand (notillustrated) whose position is controlled.

The grinding machine used for grinding a tool according to the presentinvention functions under a four-axis control as illustrated in FIG. 7and FIG. 8, that is, the grinding machine has functions with which tomove the chuck in the X axis and Y axis directions, with which to rotatethe chuck 11 around the axial center O of the chuck 11, and with whichto rotate the chuck 11 in the b axis direction illustrated in FIG. 8(not to move the chuck 11 in the Z axis direction). These functions ofthe grinding machine enable grinding of the flank face, the rake face,and a rising surface, described below, with one continuous movement ofthe chuck without having to regrip the tool.

The rake face 5 is ground while the chuck 11 holding the cubic boronnitride cutting tool 1 is rotated in a desired direction and the rakeface of the edge tip 3 is pressed against the end surface of thegrinding wheel 12 so as to be parallel to the surface, as illustrated inFIG. 9.

At this time, the edge tip 3 is ground so as to be slightly inclinedwith respect to the end surface of the grinding wheel 12, so that therake face 5 can be provided with a positive rake angle.

In this invention, a positive rake angle is provided to a lathe tool fora heat-resistant alloy.

It has been common for a cutting tool made of a material having a hightoughness, such as cemented carbide or high speed steel, to be providedwith a positive rake angle at its cutting edge.

However, it has not been common for a cubic boron nitride cutting toolused for lathe turning of a heat-resistant alloy to have a positive rakeangle.

The first reason for this is that it has been regarded as important fora cutting edge made of a sintered cubic boron nitride compact to have ahigh chipping resistance when used for the purpose of heat-resistantalloy cutting.

The second reason is that a positive rake angle causes a decrease inedge temperature in accordance with the enhancement of the sharpness.

Specifically, the decrease in edge temperature is unintended for thecutting method, involving selection of a sintered cubic boron nitridecompact having a low thermal conductivity as a cutter for cutting aheat-resistant alloy and cutting of a workpiece while softening theworkpiece with cutting heat occurring in accordance with an increase inedge temperature caused by an increase in cutting speed, and thedecrease in edge temperature has been thought to reduce the effect ofpreventing chipping of the cutting edge.

There has been no finding that the life of a cutting tool can beprolonged by forming a positive rake angle in a cubic boron nitridecutting tool used for cutting a heat-resistant alloy. The inventors havecoincidently found the effect after repeating various tests and seekingfor the way to prolong the life of the tool.

The rake face 5 having a flat surface is preferable because such a rakeface has a high workability. When the rake face 5 provided with apositive rake angle is formed, a step is formed between the surfacelowered by being ground and an unground surface.

It is preferable that the portion at which the step is formed be formedinto a rising surface 7, which is curved when the tool is viewed in aplan, and the rising surface 7 is continuous with the terminal end ofthe rake face 5 (end farther from the cutting edge).

The curved rising surface 7 can be processed using a side surface of thegrinding wheel 12. The grinding wheel 12 preferably has a radius withinthe range of approximately 50 mm to 200 mm. The rising surface 7 groundusing a side surface of such a grinding wheel is formed into a curvedrising surface having a radius of curvature R of 50 mm to 200 mm. Inaddition, an inscribed circle interior to a corner portion at which therising surface 7 is connected with the terminal end of the rake face 5,has a radius r within the range of approximately 5 μm to 50 μm, wherebycurling of chips is facilitated.

Grinding of the rake face 5 lowers, to a certain degree, the foremostend (edge) of the cutting edge 4 toward the bottom surface of the basemetal with respect to the line extended from a flat upper surface 2 a ofthe base metal 2 (the same level as the upper surface 2 a) in a crosssection taken along the bisector CL of the corner angle as illustratedin FIG. 4. The degree d of axial lowering (the amount by which the edgeis lowered with respect to the line extended from the upper surface 2 a)preferably falls within the range of 10 μm to 100 μm.

The lower limit of the degree d of axial lowering is limited to 10 μm sothat a sharp cutting edge can be formed after grinding the rake face.

On the other hand, as the degree d of axial lowering increases, an extraamount by which the rake face is unnecessarily ground increases. Inaddition, as the degree d of axial lowering increases, the cuttingresistance also increases. With these increases taken intoconsideration, an allowable upper limit of the degree d of axiallowering is approximately 100 μm.

The cutting edge 4 lowers to a larger degree with respect to the lineextended from the upper surface of the base metal with increasingdistance from the foremost end (edge). Because of this, the rake angleat each portion of the cutting edge at the back of the foremost end(rake angle that appears in a cross section taken along the lineperpendicular to each portion of the cutting edge in a plan view) issmaller than the rake angle at the foremost end; when, for example, therake angle at the foremost end illustrated in FIG. 4 is 10°, the rakeangle at each portion of the cutting edge at the back of the foremostend is 7° or 8°.

Desirably, the cutting edge 4 has a higher strength at a position awayfrom the main portion (edge) than at the edge. Making the rake face flatcan also satisfy such a demand.

By grinding the rake face 5 with the method illustrated in FIG. 9,ground streaks 8 substantially perpendicular to the bisector CL of thecorner angle when the tool is viewed in a plan are formed on the rakeface 5 as illustrated in FIG. 3. The ground streaks 8 extending in thatdirection are effective in preventing chips from adhesion on the rakeface 5. Strictly speaking, the ground streaks 8 are curved at the radiusof curvature approximate to the outer diameter of the grinding wheel 12.Thus, the ground streaks 8 are described as being substantiallyperpendicular to the bisector CL herein.

The cubic boron nitride cutting tool according to the present inventionforms a sharp cutting edge by forming a positive rake angle. Since sucha sharp cutting edge is easily chipped, it is preferable for the cuttingedge to be subjected to a fine R honing for toughening the edge, asnecessary.

The R honing is preferably performed so as not to weaken the effectproduced by making the rake angle positive (that is, enhancing thesharpness). The R honing at the radius of curvature within the range of0.005 mm to 0.02 mm can satisfy this demand.

EXAMPLE

Cubic boron nitride cutting tools having specifications illustrated inTable I were prototyped and their performances were evaluated. Theprototyped tools have the shape as illustrated in FIG. 1. Table I alsoshows the thermal conductivity of the edge tip and the positive rakeangle at the cutting edge of the edge tip of each of sample Nos. 1 to10. Table II shows the composition of the edge tip of each sample shownin Table I.

For the sintered cubic boron nitride compacts forming the edge tips,cubic boron nitride particles having an average particle diameter of 1μm have been used. However, cubic boron nitride particles having averageparticle diameter within the range of 0.5 μm to 2 μm can form sinteredcubic boron nitride compacts that negligibly have a big difference inperformance.

The sample Nos. 1 to 3 and Nos. 7 to 10 satisfy the specificationsaccording to the present invention in terms of both the thermalconductivity of the edge tip made of a sintered cubic boron nitridecompact and the degree of the positive rake angle. The sample Nos. 4 to6 are out of the specifications according to the present invention interms of the thermal conductivity of the edge tip.

The thermal conductivity of the edge tip differed between the sampleNos. 1 to 4 and Nos. 7 to 10 by adjusting the cubic boron nitridecontent. Commercially available Borazon (the product name from DiamondInnovations) was used as the edge tip of the sample No. 5. Acommercially available β-SiAlON ceramic tool was used as the sample No.6.

Using these prototyped tools, workpieces, which are Inconel 718 barshaving a hardness of HRC 46, a diameter of 120 mm, and a length of 250mm, were cut under the following conditions:

Cutting Conditions

-   -   Cutting Speed V: 200 m/min    -   Feed f: 0.1 mm/rev    -   Depth of Cut ap: 0.3 mm    -   Coolant: 20-fold dilution of emulsion.

In the evaluation test, the life of the tool was determined at thearrival of either VN or VB at 0.2 mm and the length of cut up to the endof its life was examined. The results are shown also in Table I.

TABLE I Thermal Lifetime Conductivity Rake Arrival at Cutting Cut LifeSample of Edge Angle Length of 500 m Length Determining No. Tip (W/m ·K) (°) VN (mm) VB (mm) (km) Factor 1 20 10 0 0.093 1.58 VB 2 50 10 00.076 1.95 VB 3 70 10 0.063 0.073 1.55 VN 4 10 10 0 0.134 0.87 VB 5 10010 0.138 0.064 0.77 VN 6 15 10 Broken Broken Unable to Measure due toInitial Chipping 7 50 2 0.085 0.067 1.22 VN 8 50 5 0 0.079 1.91 VB 9 5015 0 0.074 1.89 VB 10 50 20 0.070 0.063 1.40 VN

TABLE II Composition of Edge Tip Cubic Boron Sample Nitride ContentBinder Phase No. (vol %) Component 1 63 WC—Co—Al 2 78 WC—Co—Al 3 83WC—Co—Al 4 53 WC—Co—Al 5 88 WC—Co—Al 6 — — 7 78 WC—Co—Al 8 78 WC—Co—Al 978 WC—Co—Al 10 78 WC—Co—Al

As is clear from this test results, the cutting tools having the thermalconductivity of the edge tip made of a sintered cubic boron nitridecompact within the range of 20 to 70 W/m·K have prolonged their lives toa larger extent than the sample No. 4 whose thermal conductivity is 10W/m·K or the sample No. 5 whose thermal conductivity is 100 W/m·K. Thelife prolonging effect particularly significantly appears in the sampleshaving a positive rake angle within the range of 5° to 15°.

Cutting tools to which this invention is applied may have any shape,such as a rhombus, a triangle, a square, or a polygon having five ormore corners. In addition, the present invention is applicable to allthe cutting tools having an edge tip made of a sintered cubic boronnitride compact at at least one corner portion of a base metal.

REFERENCE SIGNS LIST

-   -   1 cubic boron nitride cutting tool    -   2 base metal    -   2 a flat upper surface    -   3 edge tip    -   4 cutting edge    -   5 rake face    -   6 flank face    -   7 rising surface    -   8 ground streak    -   θ rake angle    -   CL bisector of corner angle    -   d amount by which foremost end of cutting edge is lowered with        respect to line extended from base metal upper surface    -   11 chuck    -   12 grinding wheel    -   O axis of chuck

1. A cubic boron nitride cutting tool, comprising: an edge tip made of asintered cubic boron nitride compact including cubic boron nitrideparticles; and a base metal that holds the edge tip at a corner portionof the base metal, wherein: a cutting edge formed on the edge tip of thetool has a positive rake angle; the edge tip has a rake face, a risingsurface, and a top surface; in a cross section taken along a bisector ofa corner angle of the corner portion; a first end of the rising surfaceis connected with the top surface and a second end of the rising surfaceis connected with the rake face; and in a top view, (i) the rake facehas grinding lines having a concave curve that faces the corner portionof the tool, and (ii) the grinding lines are parallel to the risingsurface.
 2. The cubic boron nitride cutting tool according to claim 1,wherein a degree of the positive rake angle is within the range of 5° to15°.
 3. The cubic boron nitride cutting tool according to claim 1,wherein, in a cross section taken along a bisector of a corner angle ofthe corner portion, a foremost end of the cutting edge is lowered by 10μm to 100 μm toward a bottom surface of the base metal with respect to aline extended from a flat upper surface of the base metal and thus thecutting edge is lowered to a larger extent with increasing distance fromthe foremost end.
 4. The cubic boron nitride cutting tool according toclaim 3, wherein a flat rake face is formed in the edge tip.
 5. Thecubic boron nitride cutting tool according to claim 1, wherein a risingsurface curved at a radius of curvature R of 50 mm to 200 mm iscontinuous with a terminal end of a rake face of the edge tip formingthe positive rake angle.
 6. The cubic boron nitride cutting toolaccording to claim 1, wherein the sintered cubic boron nitride compacthas a thermal conductivity within the range of 20 to 70 W/m·K.
 7. Thecubic boron nitride cutting tool according to claim 1, wherein the cubicboron nitride particles have an average particle diameter within therange of 0.5 μm to 2 μm.